Alu-derived (SIRLOIN) Elements Promote Nuclear Localization of Long Noncoding RNAs and mRNAs in Human Cells
Figure 1 – Experimental Procedure for Examining Effects and Functions of NucLibA and NucLibB Libraries and HNRNPK in RNA Localization and Expression inside MCF7 Cells.
Abstract
The subcellular localization of messenger RNAs (mRNAs) and long noncoding RNAs (lncRNAs) determines their functions, and both play important roles in cellular processes. Nevertheless, the molecular mechanisms controlling RNA localization are still not well understood. In a recent study [1], Yoav Lubelsky and Igor Ulitsky report the identification, through high-throughput screening, of SIRLOIN elements, a class of repeated sequences generated from Alu repeats that facilitate transcript nuclear localization. Transcripts hosting these elements are lowly expressed and are enriched among lncRNAs. Through mutagenesis experiments, it was shown that pyrimidine-rich motifs inside SIRLOIN elements are essential for HNRNPK binding and nuclear enrichment. Furthermore, the involvement of HNRNPK in promoting nuclear localization was validated by knockdown investigations. These results demonstrate the functional importance of SIRLOIN elements in cellular activities and provide insights into the molecular mechanisms controlling RNA subcellular distribution.
Review
Without producing proteins, long non-coding RNAs (lncRNAs) are essential regulators of gene expression, controlling a wide range of cellular functions. These are transcripts longer than 200 nucleotides originating from parts of the genome that were previously considered as “junk” DNA. Their complex functions include post-transcriptional regulation, chromatin organization, epigenetic modification, nuclear architecture, developmental processes, and implications in human disease [2]. Gaining insights into the complex functions of lncRNAs is important to reveal fundamental principles of cellular biology and provides opportunities for novel treatment approaches to a wide range of illnesses. Moreover, the evolutionary path of lncRNAs in vertebrates highlights the dynamic interaction between the acquisition of new roles through molecular adaptations and the conservation of vital activities, highlighting the importance of lncRNAs in forming biological complexity [3]. The main goal of the authors’ research was to identify elements within lncRNAs and mRNAs that can drive nuclear localization.
NucLibA Library
Through the development and examination of the NucLibA library, the authors of the study used a methodical approach to look into the nuclear enrichment mechanisms of lncRNAs. This library consists of 5,511 sequences (tiles), each 109 nucleotides (nt) long, that are generated from 13 3′ untranslated regions (UTRs) of mRNAs enriched in the nucleus in mouse liver, 4 homologues of the abundant nuclear lncRNA MALAT1, and the exonic sections of 37 human lncRNAs [4][5]. All these RNAs were selected due to their biological importance and considering their high abundance and specific localization within the nucleus. The screening of tiles promoting nuclear accumulation was conducted by incorporating the selected sequences into the untranslated sections of Aequorea coerulescens (Ac)GFP mRNA (Figure 1) and by performing a pooled transfection into MCF7 cells. Next, they carried out a subcellular RNA fractionation from whole cell extract (WCE) and then, thanks to RT-PCR, RNA-sequencing and normalized counts of unique molecular identifiers (UMI counts), they were able to evaluate the effects of these tiles on localization and expression level (Figure1). NucLibA library high-throughput screening identified 19 sections from 14 genes, spanning 2-4 overlapping tiles, linked to more than 30% nuclear enrichment. The highest enrichment was seen in tiles derived from the lncRNAs JPX, PVT1, and NR2F1-AS1 [1]. The overlap region between the most active patches converged on a 42 nt fragment that contained three stretches of at least six pyrimidines (C/T) and these tiles were also found to overlap Alu repeat sequences. For this reason, authors named the 42 nt sequence “SIRLOIN” (SINE-derived nuclear RNA LOcalization). Additional investigations, utilizing the PrimeFlow RNA assay in imaging flow cytometry and quantitative PCR with reverse transcription (qRT–PCR), confirmed the nuclear enrichment activity of SIRLOIN-containing tiles. The authors concluded that each SIRLOIN-containing tile independently determine nuclear enrichment of transcripts and have an impact on expression levels as well as localization (Figure 1). Furthermore, the correlation between SIRLOIN elements and nuclear enrichment was validated by RNA sequencing data of subcellular fractions obtained from ENCODE database [6], indicating that this process is ubiquitous throughout different species [3].
NucLibB Library
By building a second library, the NucLibB library, the authors deepened their examination of RNA localization mechanisms. This library was built as an extension of NucLibA and contained modifications of the most efficient tiles from NucLibA as well as extra RNA sequences that overlapped with SIRLOIN elements. These sequences were incorporated into the 3′ untranslated region of (Ac)GFP mRNA (Figure 1). 18 additional SIRLOIN-overlapping regions from five lncRNAs and five mRNAs were found using NucLibB screening, underscoring the significance of SIRLOIN elements in promoting nuclear accumulation [1].
Furthermore, distinct sections from the same transcript demonstrated substantial nuclear enrichment in SIRLOIN-matching tiles, indicating persistent action of these elements in mediating nuclear localization.
Interestingly, mutagenesis experiments revealed how important particular nucleotide patterns are to the activity of SIRLOIN elements. Lubelsky and Ulitsky also looked into how SIRLOIN elements were dependent on context, and they found that sequences nearer the 3′ end were associated to higher nuclear enrichment and lower expression levels. All of these results pointed to the critical function of SIRLOIN elements in nuclear localization mediation.
RNA-binding protein: HNRNPK
Next, the authors hypothesized that the nuclear enrichment of transcripts containing SIRLOIN motifs is mediated by interactions with specific RNA-binding proteins. To test this hypothesis, they searched ENCODE eCLIP datasets for protein-RNA interaction sites that were particularly enriched in SIRLOINs. Out of the 112 candidate proteins that were examined, HNRNPK came out as the top enriched RNA binding protein. Moreover, motif analysis showed that the RCCTCCC sequence, which was found as essential for mediating localization inside SIRLOIN elements, closely matched the RNA-binding preferences of HNRNPK, revealing its affinity for pyrimidine-rich sequences with a CCTCC core. Additionally, the binding of HNRNPK to (Ac)GFP mRNA enriched with SIRLOIN elements was verified by experimental validation employing RNA immunoprecipitation (RIP) using an HNRNPK-specific antibody [1]. By tethering HNRNPK to the 3′ UTR of a luciferase mRNA using a λN peptide-BoxB system, the authors were able to further clarify the direct causative role of HNRNPK in promoting RNA nuclear enrichment. Furthermore, by the examination of HNRNPK binding clusters in eCLIP data from several cell lines, the scientists observed a broad influence of HNRNPK binding on RNA localization. Interestingly, higher nuclear enrichment of lncRNAs and mRNAs was correlated with the quantity of HNRNPK eCLIP binding clusters. Ultimately, HNRNPK’s function in controlling subcellular localization was further confirmed by knockdown studies employing small interfering RNAs (siRNAs). These experiments revealed notable changes in the nuclear-to-cytoplasmic ratio of hundreds of genes [1]. All of these results point to the same conclusion: HNRNPK binding promotes nuclear enrichment of transcripts with SIRLOIN motifs (Figure 1), offering mechanistic insights into how RNA localization is regulated in cells.
Conclusions
In conclusion, the thorough analysis carried out in the authors’ work clarifies the complex mechanisms controlling the nuclear enrichment of some mRNAs and lncRNAs. Through the construction and analysis of the NucLibA and NucLibB libraries, the scientists discovered a common process for RNA subcellular distribution across species, identifying Alu-derived SIRLOIN elements as important determinants of nuclear localization. Moreover, the clarification of the function of HNRNPK in facilitating nuclear enrichment offers molecular understandings into the regulation of RNA localization. The results highlight the role that RNA-binding proteins and sequence elements play in coordinating the sub-cellular localization of transcripts. Subsequent studies may investigate different RNA-binding proteins and sequence motifs related to RNA intracellular distribution. Additionally, they may clarify the functional implications of RNA localization anomalies in illness conditions. Deeper understanding of the regulatory networks controlling gene expression and cellular physiology may also be gained from research clarifying the dynamic interaction between RNA distribution and cellular signaling pathways [2]. Overall, these findings open up new possibilities for therapeutic approaches that target RNA localization mechanisms in disease situations, and pave the way for a more thorough understanding of RNA biology.
References
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